This invention relates in general to fuel cell components, and in particular to polymer-containing fuel cell components such as polymer electrolyte membranes and electrodes. The polymers can include both proton conducting polymers and non-proton conducting polymers.
Fuel cells are a promising technology for generating electricity with higher efficiency and lower emissions than most current methods. Polymer electrolyte membrane (“PEM”) fuel cells include a proton conducting polymer membrane sandwiched between an anode and a cathode. A fuel such as hydrogen or methanol is flowed into contact with the anode where it dissociates into electrons and protons. The electrons, which cannot pass through the membrane, flow from the anode to the cathode through an external circuit containing an electric load, which consumes the power generated by the cell. On the opposite side of the cell, the cathode adsorbs oxygen from the air, generating a potential that pulls the electrons through the external circuit to give them to the adsorbed oxygen. When an adsorbed oxygen receives two electrons it forms a negatively charged oxygen anion. The polymer electrolyte membrane allows the protons to diffuse through the membrane. When two protons encounter an oxygen anion they join together to form water. In addition to polymer electrolyte membranes, proton conducting polymers can also be used in other fuel cell components. For example, they can be used as binders along with particles of carbon-supported catalyst in the preparation of electrodes for fuel cells.
Heteropolyacids (“HPAs”) are proton conducting solids often used as additives with a polymer electrolyte membrane to improve the conductivity of the membrane. Unfortunately, HPAs are highly soluble in water and as a result, if they are added by mixing with a proton conducting polymer to prepare a polymer electrolyte membrane, they may be washed away from the membrane during fuel cell operation over a period of time. This may adversely affect the performance of the fuel cell. HPAs with low water solubility such as zirconium hydrogen phosphate have been explored to make polymer composite membranes.
The literature describes composite polymer electrolyte membranes made with HPAs, polymer, and an inorganic material. The composite membranes reported in the literature are either made by a sol-gel process [Grot, W. G.; Rajendran, G. in PCT Int. Appl.; (Du Pont, USA, WO 96/29752, 1996] or by direct mixing of inorganic filler to a polymer solution [Nunes, S. P.; Ruffmann, B.; Rikowski, E.; Vetter, S.; Richau, K. J. Membr. Sci 2002, 203, 215-225]. The sol-gel process leads to uniform distribution of inorganic particles in the polymer matrix. However, controlling the ratio of polymer to inorganic filler is difficult. The direct mixing process adequately controls the amount of inorganic filler in the polymer matrix but it is very difficult to obtain homogeneous distribution of inorganic particles. Furthermore, the particle size obtained from this procedure is large and as a result the membranes do not have adequate strength.
M. L. Poncea, L. A. S. de A. Pradoa, V. Silva, S. P. Nunes Desalination 162 (2004) 383-391, describes organic-inorganic membranes for direct methanol fuel cell application prepared from sulfonated polyether ether ketone, containing heteropolyacids and an oxide phase either generated by hydrolysis of amino-modified silanes or by dispersion of surface-modified fumed silica. The heteropolyacid contained epoxy groups that reacted with the amino-groups in the oxide phase. The reaction provided a covalent bond between the heteropolyacid and the insoluble oxide phase, resulting in its fixation in the membrane.
Ramani et al (Electrochimica Acta 50 (2005) 1181-1187) describes a method for making water insoluble HPA by ion exchanging protons of HPA with cations such as ammonium, cesium, rubidium and thallium. The water insoluble additives are formed first and then they are added to PEM. The particle size of the additives dispersed in the PEM is around a few microns. Furthermore, a 5 weight percent (wt %) loss of these additives occurs in aqueous media.
It would be advantageous to provide improved fuel cell components including polymers and heteropolyacids.